Posted
by
simoniker
on Thursday December 11, 2003 @04:37AM
from the mr-bigglesworth-looks-on dept.

Smitty825 writes "After slowingdown light to slow speeds, scientists at Harvard University have been able to stop light for a very brief period of time without destroying its energy. The article explains how it is different from this previous light-stopping science story - this will hopefully help the development of quantum computers and ways to communicate over long distances without being eavesdropped on."

BBC News [bbc.co.uk] has an article which speaks a bit more to Quantum crytography.

"Quantum cryptography might provide very secure forms of electronic encryption, because the process of eavesdropping on an electronic message would introduce errors in the message, garbling it."

"This would allow you to exchange a key on a public channel, but whereas any classical system can be broken by an eavesdropper, in quantum cryptography you would always find out if someone was looking at your message," Professor Zubairy told BBC News Online."

The light beam is stored (in gas atoms) rather than stopped. It's a bit like sending an e-mail - you don't get the same electrons that were sent to you from the other person's computer, but the electrons that come down your telephone line/DSL/cable are identical in every respect.

That just means that if you were, in fact, able to drive your car through their rubidium medium, it might produce somthing akin to cherenkov radiation, another example of massive particles traveling faster than c/n.

I think you're not supposed to be able to go faster than light under the _same conditions_. If someone used forward and backward control beams to time-vary your Rabi-frequencies, I doubt you'd be going anywhere fast.

but there's a stronger concept at work in this light thing too, indistinguishability. Sometimes, the 'same' photon doesn't mean anything. actually, it never means anything. photon is the observed quanta, and you can only ever make the observation once, same with electrons.

That may be how traditional optical communications works. Quantum crypto, otoh, relies on the light being put in a certain polarization state by the sender. It's designed so that a stream of single photons go from sender to receiver; there can be no equipment in-between. If an intermediary views this photon en-route, it disturbs the polarization seen by the receiver. Because of the way the sender and receiver can agree on which photons were correctly measured, any aberrations (intercepted photons) are discarded. The most you can hope for is a denial-of-service.

They're not stopping the photon. They're simply storing it in several atoms quantum spin. Then they hit it again with a laser and get the earlier pulse back out of the quantum spin stored in the atoms. It's rather limited because, quoting from Science News

So far, Hau and her team report the longest storage time for pulses--about a millisecond. By then, random atomic motion had washed out most pulse information, the researchers suspect. The Harvard-Smithsonian team reports that its pulses' information is erased partly because atoms escape from the region lit by the coupling laser.

However your post should be modded funny, because it's a witty, clever response rather than the usual worn jokes which somehow seem to get modded up all too frequently.

Reminds me of a childrens story I read once about a time machine, which was based on a nutty inventor who managed to build a car that got progressively faster. First of all it took a minute to get a specific distance, then 30 seconds, then 1 second, until in fact it took no time at all and then less than no time to get there until it ended up travelling backwards in time...

(I believe you can use the standard deviation as the uncertainty here.) This "law" that results from our model for quantum mechanics thus tends to put a limit on how fast a quantum/optical computer can be.

"this will hopefully help the development of... ways to communicate over long distances without being eavesdropped on"

We already have that. Light based fiber runs are impossible to tap into without having to break the connectivity to hook up an additional device. Of course, nothing goes coast-to-coast directly, so they're plenty of chances for the spooks to install their logging equipment at a switching station or router.

The only way to communicate securely without encryption is to totally control physical access to the line, which just plain isn't gonna happen over long distances.

no, that's false. the universal speed limitation is the speed of light in a vacuum. Because light passing through matter moves slower than it does through a vacuum, it's perfectly possible to move faster than the "local" speed of light. Physicists have studied this by firing high-speed particles into crystals. Basically the particle creates a shockwave, a sort of optical equivalent of a sonic boom. It's called Cherenkov radiation if you want to look it up.

Some people have posted claims that this is similar to the earlier experiments of Lene Hau, where the light pulse was indeed stored as excitations in trapped atoms (either in a BEC as in Hau's case, or in a vapor cell as in Lukin's earlier experiment).

This is quite different from what's going on here. In this experiment, two lasers are used to polarize the atomic vapor as a function of position, and then bouncing light off that polarization gradient. Think of what happens when you put light in between two highly reflecting mirrors, and let it bounce back and forth. Then think about what happens if you nest thousands of these mirrors within each other, so that if the photon leaks out of one, it has to deal with the next one, only one wavelength away. Since the photon is spending so much time bouncing back and forth, it doesn't really have a chance to escape the gas, and so we say that it's trapped.

He stated the 'color' of our photo receptors. Although our photo receptors pick up C, Y, and M -- it is because their colors (as he said) are R, G, and B.

ie: the Red Photoreceptor reflects RED, that is why it is a red colored photoreceptor. Since it reflects RED, it picks up BLUE and GREEN, which make one of the (secondary) colors you mentioned.

You are also wrong saying that RGB is used for pigment. Pigment gets its color by absorbing color, and you see what is reflected. RGB is used for TVs and Monitors where there is a direct source of light.

The speed of light is constant. What they are doing is having photons absorbed by atoms in the medium which are later emitted. The velocity at which the photons travel is constant.

What they've really done is created a medium which slows the asorption and emittion of the photons so drastically that it is descernable by the naked eye. What they have NOT done is altered C. In other words, what they've done is the equivalent to shining a flash light through water and saying they've slowed the speed of light. This is drastically different from actually slowing the velocity of the photons through a vaccuum. In their case, their medium is constantly absorbing and re-emitting the photons. In essense, the photons that emerge from the other end are not the same photons that entered it. They are equivalent copies. Such is what happens with glass or any other transparent medium.

The "fact" that fiber optic cable is untappable is more a article of faith rather than objective reality.
Providing you have physical access to a optical cable it is trivial to tap one. All that is required is that the cable be bent. The bend refracts light through the cladding and it can be detected with a suitable detector. The loss increases on the cable as a result of the bend (but communications are not disrupted so long as the loss is less than the loss budget for the link. Most carriers have test equipment based on this principle to allow them to perform non-invasive testing of their fiber plant.
Hence the problem is more of an access issue. BTW this is why secure F/O cables run within a pressurized conduit. A pressure drop indicates someone is attempting to gain access OR a squirrel is chewing through it!.
Yes I design F/O cable plant!

Energy = h*f, where f is the frequency of light and h is Planck's constant. The percieved color of slowed light is the same as when it is at speed c, if it has the same energy.

Normally, we say that the color of light depends on the wavelength of the light, but technically it depends only on the frequency (not the same thing!). We know that:

frequency = speed of light / wavelength

But when light passes through any transperent material (with index of refraction n > 1), the speed of light changes, as well as the wavelength. Coincidentally, they also change by the same amount.

speed of the light = c/n
wavelength of light = L/n
where c and L are the speed and wavelength of the light in a vacuum and n is the index of refraction

Because they change by the same amount, the frequency remains unchanged. At least, this is how it works in normal transperent materials like water, air, and glass. This cutting edge stuff may be different, but the article lacks all the good technical bits for me to be able to tell.